1. Field of the Invention
The present invention relates to a failure diagnostic device of an evaporative gas purge control system for diagnosing a presence of a drain valve sticking provided in a fresh air line to a canister.
2. Description of the Related Art
Some engines for vehicles have been provided with an evaporative gas purge control system to feed (or return) an evaporative fuel to an engine intake system and burn thereof in order to prevent an evaporative fuel gas generated in a fuel tank from leaking outside.
Such a kind of the evaporative gas purge control system like this comprises a canister to adsorb the evaporative fuel, an evaporative gas passage to communicate the canister with the fuel tank, and a purge passage to communicate the canister with the engine intake system, allows the evaporative fuel generated in the fuel tank to be adsorbed by the canister, and feeds the evaporative fuel to the engine intake system by utilizing a negative pressure generated in the engine intake system under certain. operating conditions, and burns thereof.
In the evaporative gas purge control system, a leak hole is formed in the evaporative gas purge system leading from the fuel tank to the engine intake system, or when a seal of a joining part of each passage is degraded, the evaporative fuel leaks into the atmosphere from these places, and a failure diagnosis device to check a presence of any leak from the leak hole or the like is added thereto.
For example, in Japanese Unexamined Patent Application Publication No. 9-264207, the evaporative gas purge control system has been disclosed, in which a pressure control valve to maintain the pressure in the fuel tank at a specific value is interposed in an evaporative gas passage to communicate the fuel tank with the canister, and a purge control valve to open and close the purge passage is interposed in the purge passage to communicate the canister with the engine intake system, and a drain valve is disposed in a fresh air introducing port of the canister.
When performing a failure diagnosis in such a known evaporative gas purge control system, first, the drain valve is closed and the pressure control valve is opened while the purge control valve is opened to introduce the negative pressure generated in the engine intake system into the fuel tank to set the pressure in the fuel tank to be negative. Then, the purge control valve is closed, a passage from the fuel tank to the purge control valve is closed, and a pressure rise in the fuel tank is measured in thereof state.
The presence of any evaporative fuel leakage in the evaporative gas purge system is determined on the basis of the degree of pressure changes calculated in accordance with the difference between the pressure in the fuel tank measured immediately after the purge control valve is closed (hereinafter, referred to as “tank internal pressure”) and the tank internal pressure after an elapse of a predetermined time.
The pressure control valve interposed in the evaporative gas passage is provided to maintain the pressure in the fuel tank at the specific value, and to prevent any abnormal drop of the tank internal pressure by the negative pressure from the engine intake system during the evaporative gas purge control.
Therefore, the pressure control valve comprises a valve chamber interposed in the evaporative gas passage, a reference pressure chamber communicated with the atmosphere, a diaphragm having a valve element fixed to demarcate (or separate) the chambers and open and close the valve chamber, and a diaphragm spring disposed on the reference pressure chamber to press (or push) the diaphragm for a closing direction, and further comprises a so-called diaphragm valve in which the valve element is opened if the tank internal pressure is higher than a resultant force of the atmospheric pressure flowing into the reference pressure chamber and a diaphragm spring force, and the evaporative fuel generated in the fuel tank is adsorbed by the canister.
However, since a reference pressure chamber of the pressure control valve is opened to the atmosphere, the evaporative fuel leaking into the reference pressure chamber can be discharged outside.
In this case, a discharge of the evaporative fuel from the reference pressure chamber can be prevented by communicating the reference pressure chamber with the canister, and allowing the evaporative fuel leaking to the reference pressure chamber side to be adsorbed by the canister.
However, if the fresh air communicating port of the pressure control valve is communicated with the canister, the internal pressure in the canister is introduced in the reference pressure chamber. Therefore, if the drain valve to open/close, for example, the fresh air introducing port is close-stuck, the low internal pressure in the canister is introduced into the reference pressure chamber during the evaporative gas purge control, the diaphragm is attracted to the reference pressure chamber side, and the pressure control valve is opened. As a result, the evaporative fuel in the fuel tank is sucked into the canister side, and the tank internal pressure is considerably dropped.
When the tank internal pressure is dropped, the tank internal pressure can not be raised to a start pressure of the diagnosis during regular failure diagnosis for performing a leak determination of the evaporative gas purge system, failure chances of the diagnosis are reduced, and the diagnosis accuracy is degraded.
For example, the above Japanese Unexamined Patent Application Publication has disclosed a technology to perform the failure diagnosis to check the presence of any closed sticking of the drain valve after the failure diagnosis to check the presence of any evaporative fuel leakage in the evaporative gas purge system.
In other words, after completing the leak determination in the evaporative gas purge system, the drain valve is opened from this state, the pressure control valve is closed, and measured is the pressure rise of the tank internal pressure in the state. If the rate of this pressure rise is smaller than a reference rate, it is determined that the drain valve is close-stuck.
Further, the pressure control valve has a reed valve to be opened when the pressure on the canister side is higher than the pressure on the fuel tank side. Because the atmospheric pressure is introduced by a pressure difference in the fuel tank via the reed valve if the drain valve is normally opened, the pressure in the fuel tank is raised in a relatively earlier stage. On the other hand, when the drain valve is close-stuck, little differential pressure is caused between the fuel tank side and the canister side. Therefore, no pressure is introduced from the canister side to the fuel tank side, and the tank internal pressure is raised only by the pressure corresponding to the fuel evaporation. The pressure is raised less, and in such a case, it is determined that the drain valve is close-stuck.
However, as described above, if the reference pressure chamber of the pressure control valve is communicated with the canister, the tank internal pressure is considerably dropped during the evaporative gas purge control, and chances for an ordinary failure diagnosis to check the presence of any evaporative fuel leakage are reduced. Further, the chances for the failure diagnosis to check any closed sticking of the drain valve are also reduced, and as the result, a detection accuracy of the failure diagnosis is degraded.